In digital printing systems, a picture element (pixel) may be defined as a space on a page into which dots of ink, dye or toner are placed. The placing and number of the dots determines how a human eye integrates the dots and picture elements into an image. Colors of pixels, for example, are generally controlled by the number of dots of different colors of ink within the space. A pixel having a high concentration of yellow dots will appear mostly yellow. The yellow can be altered to appear greener or more orange by the addition of dots of the other colors.
Generally, color imaging processes use subtractive coloring to achieve color images. In digital display systems, the substrate upon which an image is displayed, the screen, is generally considered to be black and colors are added together to form color images. In digital printing, the substrate is generally white, so colors are added together to subtract color from white. The subtractive colors are cyan, magenta and yellow (CMY).
In dye-based color printing systems, the dyes have high purity and intensity and the color black may be recreated on the paper by mixing the three colors together. In ink and toner-based systems, however, that is not usually the case. The mix of CMY generally results in an intermediate brownish color, not black. Therefore, black is added to the available inks and toners, and is represented by the color K.
When an image is produced using CMYK, the addition of black requires that the other color amounts be altered to account for the addition. This process is generally referred to as gray component replacement (GCR) or under color removal (UCR). Most GCR systems rely upon a series of equations that are applied to the current CMYK values of a given pixel. These equations generally only use the minimum value of the CMY values for the current pixel. This may be thought of as performing a one-dimensional GCR process.
For print mode, this one-dimensional method of GCR seems sufficient for the most applications. However, for such applications as the copy mode of multi-function device, where the electronic scan image is a mix of text, pictorial and graphic contents, the one-dimensional GCR process may result in objectionable artifacts in the final output hardcopy, such as graininess in highlight areas, color fringing, texture contouring, edge blurring, and reducing of color in dark areas.